It is particularly important to clarify the correlation between fine particulate matter (PM_2.5) and O₃ (P-O) in account of the seemingly increasing surface ozone and decreasing PM_2.5 trends in China since 2014.Therefore,observations of hourly PM_2.5 and O₃ data for Beijing and Nanjing during 2014 to 2019 were summarized to reveal their P-O correlations.The observations showed a decreasing trend of annual average PM_2.5 mass concentration,which was -6.86 and -6.15 μg·m^-3·a^-1 for Beijing and Nanjing,respectively.While the annual maximum daily 8-hour average O₃ (MDA8 O₃) increased by 1.50 and 1.75 μg·m^-3·a^-1 for Beijing and Nanjing,respectively.O₃ pollution became more serious in summer for both cities.For Beijing,when O₃ mass concentration was below/above 100 μg·m^-3,the increase of O₃ concentration would correspond to the decrease/increase of PM_2.5 concentration,indicating a negative/positive P-O correlation.The same varying P-O correlation existed for Nanjing,except for the different critical value of O₃ mass concentration of 150 μg·m^-3.Significantly positive P-O correlation,indicated by Pearson correlation coefficient (COR) being larger than 0.5 on daily time scale,was found in both Beijing and Nanjing during May to September.While significantly negative P-O correlation,indicated by COR value being smaller than -0.5 on hourly time scale,was found for both Beijing and Nanjing during November to February.Compared with Nanjing,monthly and seasonal P-O correlation varied more greatly for Beijing.As for the diurnal variation,there was a significantly positive P-O correlation at 16:00 in summer and a weakly positive correlation during 13:00-17:00 in spring and autumn,and a significantly negative correlation at 08:00 in spring,autumn and winter.